1. Field of the Invention
The present invention relates to a method for inducing refractive index changes in a micro/nanofiber, more particularly, to a method for fabricating Fiber Bragg gratings in a micro/nanofiber.
2. Description of the Related Art
There has been increasing interest in optical micro/nanofibers (MNFs) in recent years because of their unique properties. An optical MNF is an optical fiber in micro or nano scale that essentially consists of only fiber core, surrounded by air or other ambient medium. When light travels along the cladding-less MNF, it is tightly confined to the fiber core due to the large refractive index contrast between the fiber core and surrounding medium. A large fraction of the guided light can propagate outside the MNF as the evanescent wave, which makes it highly sensitive to the surrounding medium. The small size of the MNF also provides excellent flexibility and convenient configurability, allowing the easy manipulation of the MNF based devices with a complex topology. Many MNF based fiber devices have been developed, with important applications in the area such as refractive index (RI) sensing.
Fiber Bragg Grating (FBG) is one of the basic optical fiber components that have a wide range of applications. FBG introduces periodic variations in the refractive index (RI) in the fiber core that reflects particular wavelengths and transmits other wavelengths. However, a conventional FBG is intrinsically insensitive to external RI change as it is not directly exposed to the surrounding medium. Although such a difficulty may be alleviated by thinning or etching of the fiber after the FBG creation, the mechanical strength and durability of these sensing devices are greatly reduced, which limit the applications of these FBG based RI sensors.
Fiber Bragg gratings can be created by inscribing the periodic variation of refractive index into the fiber core using an intense ultraviolet (UV) light. However, this type of UV inscription method is limited to photosensitive optical fibers with hydrogen loading. Generally, germanium-doped silica fibers are used to fabricate FBG under this method.
The present FBG in MNF can overcome the above-mentioned difficulties and enhance the RI sensitivity because of its narrow bandwidth, small grating size and good measurement accuracy. Moreover, FBG can support the multiplexed system, showing significant advantages over other types of fiber gratings.
Another tool for FBG fabrication is an ultra-short laser such as femtosecond laser, which allows inscription of FBG in almost any type of optical fibers without photosensitivity, as disclosed in Mihailov et al., “Bragg Gratings Written in All-SiO2 and Ge-Doped Core Fibers With 800-nm. Femtosecond Radiation and a Phase Mask,” J. Lightwave Technol. 22, 94 (2004). Because of the extremely high light intensity across a small spatial region and an ultra-short interaction time, the refractive index change induced by the high power femtosecond pulses is highly localized, which supports the fabrication of strong and high spectral quality FBGs.